1. Field of the Invention
The present invention relates generally to the thermal protection of electric motors and, more particularly, to thermal protection through thermal modeling of motor heating and cooling, including modeling of rotor thermodynamics.
2. Description of Related Art
One of the primary causes of damage to electric machines generally, and to electric motors in particular is overheating. In the case of electric motors, such overheating typically results from overload or some type of electrical or mechanical failure in the machine, and can permanently destroy the motor or lead to a significant reduction in its useful life. To avoid such damage, a number of methods have been devised or proposed for limiting the temperature of electric motors, or at least for detecting and warning of a conditions likely to lead to overheating, and ultimately to failure of motors.
One such method involves the use of a thermal model for the motor to be protected. An example of this approach to motor thermal protection is disclosed in U.S. Pat. No. 4,939,437 issued on Jul. 3, 1990 to Farag et al., which is hereby incorporated by reference. In the method described in that patent, thermal protection for certain types of motor is provided by thermally modeling the motor as an analogous electrical circuit wherein the thermal resistances and capacitances of the motor stator windings and housing are used to iteratively calculate estimates for the temperatures of the windings and housing. Implementation of the method by means of a digital microcontroller affords real time modeling of motor temperature. Where temperatures near or in excess of rated limits result from the model, appropriate measures can be taken, such as proving an alarm or removing current to the motor.
However, such methods typically do not include rotor thermodynamics in their thermal model. Moreover, due to the variety of failure modes prevalent with different classes of motors, it would be advantageous if such models could simulate rotor heating as well as heating of the stator winding and housing. In particular, the failure modes of various types of motor depend upon the voltage class of the motor, lower voltage motors being generally "stator limited," meaning that failure typically results from overheating of the stator winding, while medium voltage motors are generally "rotor limited" on startup, or in a stalled or locked-rotor condition, and "stator limited" under normal running. Thus, to afford effective protection against overheating, an improved thermal model for medium voltage motors should provide an estimate of the rotor temperature, particularly during those phases of operation in which failure due to rotor overheating is most likely to occur, during startup, stall and locked-rotor conditions.